1. Field of the Invention
The present invention relates to a transistor such as a field effect transistor (hereinafter, referred to as a "FET") and a bipolar transistor, and a power amplifier.
2. Description of the Related Art
One conventionally well-known power amplifier, especially a power amplifier for amplifying high-frequency signals, is the one which employs a dual-gate FET as an active element in order to reduce current consumption at the time of low-output operation.
FIG. 15 shows the structure of a conventional high frequency power amplifier. In FIG. 15, the reference numerals 1 and 2 denote an AC power input terminal and an AC power output terminal, respectively. The reference numerals 3 and 4 represent an input impedance-matching circuit and an output impedance-matching circuit, respectively. The reference numerals 5, 6 and 7 denote a first gate voltage supply circuit, a second gate voltage supply circuit, and a drain voltage supply circuit, respectively. The reference numeral 8 represents a dual-gate FET. The reference numerals 9 and 10 represent variable negative power supplies. The reference numeral 11 denotes a positive power supply. The variable negative power supply 9 supplies a bias voltage to a first gate of the dual-gate FET 8 via the first gate voltage supply circuit 5. The positive power supply 11 supplies a power supply voltage for output to a drain of the dual-gate FET 8 via the drain voltage supply circuit 7.
An input signal is input from the AC power input terminal 1 and supplied to the first gate of the dual-gate FET 8 via the input impedance-matching circuit 3 so as to be amplified by the FET 8. The amplified signal is output from the AC power output terminal 2 via the output impedance-matching circuit 4.
Currant consumption at the time of low-output operation of the high frequency power amplifier can be reduced by regulating an output voltage of the variable negative power supply 10 and controlling a voltage supplied to a second gate of the dual-gate FET 8 from the variable negative power supply 10 via the second gate voltage supply circuit 6.
In the above-described conventional high frequency power amplifier, however, although the current consumption at the time of low-output operation can be reduced by controlling the voltage of the second gate of the dual-gate FET 8, since input/output impedance of the dual-gate FET 8 significantly varies along with changes in the voltage of the second gate, electrical mismatch occurs between the input impedance-matching circuit 3 and the output impedance-matching circuit 4.